For the large number of wire strands and the long distance of strand traction, the wire strands are erected normally with gantry pulling traction system that allows continuous pylon passing in the construction of main cables with PPWS(prefabricated parrel wire strand) method for an extra-large span suspension bridge. The accurate calculation of traction force is of realistic significance to the selection of traction equipment and ensuring smooth construction. In early days of suspension bridge construction, the capacity demand and type of traction equipment were determined mainly based on experience. In this article, exploration was made for the traction force calculation method of the gantry pulling traction system for PPWS construction, and the results were compared with measured data on work site. It proves that the calculation result is reliable and can be used in construction calculation.